At this point, you should see in the Odoo log that the site-packages directory appears in the addons path.

You can easily install additional addons. For example, should you like to have an Odoo 8 shell,
using the OCA shell module, simply pip install the module:

pip install odoo8-addon-shell
openerp-server shell

To view addon packages that are installed in your virtualenv,
simply use pip list | grep odoo<8|9|10>-addon- (note official addons
are part of the odoo package).

Technical notes

With Odoo 8 and 9 it’s not possible to make openerp.addons a namespace package
(because openerp/__init__.py contains code), we use a pseudo-package named
odoo_addons for the sole purpose of discovering addons installed with
setuptools in that namespace. odoo_addons is not intended to be imported
as the Odoo import hook will make sure all addons can be imported from
openerp.addons as usual.